i hate these proofs i never understand what they mean i cant seem to get me head around them all tips appreciated.

show that the number of integer sequences ($\displaystyle e_{1},e_{2},...,e_{n}) $such that $\displaystyle 0\leq e_{i} , i=1,2,..,n $ and $\displaystyle e_{1} +e_{2} +...+e_{n}=k $ is equal to

( n + k - 1 ) (brackets are meant to be over k aswel...

k

thanks.